Vader would be unrecognizable if Padme had lived. He turned to the Dark Side for her sake; his passion for her supplied him with the anger and the determination to reject the Jedi and challenge death. It was her death that turned him into a docile, cynical apprentice who, as far as we know, gave up his dreams and aspirations. It destroyed his spirit far more than the loss of his limbs.

If Padme had lived, Vader would almost certainly have challenged the Emperor's rule early on. He would have continued to be obsessed with her to the exclusion of obeying Palpatine. Given time, this obsession could have turned him back to the Light Side. Love can work both ways, as seen at the end of the Return of the Jedi.

Despite this, if Vader had not lost his duel to Obi-Wan in such spectacular fashion, Palpatine probably would have wanted to keep Padme alive to act as a carrot for Vader, baiting him to keep exploring the dark side of the force out of his perverted love for her. But since Vader was a broken man, the Emperor was most likely happy to use her death to snuff out his apprentice's spirit and ensure that he would never try to overthrow his master. Little did he know that Padme's son would soon come and restore Vader's lost passion, giving him a reason to be more than just an apprentice.

I think that Padme's presence would not so much enhance the Original Trilogy as preclude it. The role of Luke Skywalker would become less compelling; even unneeded.